Minors were sold alcohol three times during a Controlled Purchase Operation in Marton and Hunterville.
The operation, which was jointly run by police, the local Licensing Authority and Public Health was held on Friday, 16 January.
The long-established system uses under-age volunteers under the supervision of the police to test licensed premises and their commitment to denying sales to minors.
During the operation two volunteers aged 17 and 16 attempted 14 purchases at five off-licences, supermarkets and convenience stores and four bars.
They were jointly sold alcohol at one bars and each made a successful purchase at two different off-licences.
Constable Karl Williams of Marton Police said: "We hadn't run an operation of this kind locally for some time so were not sure what to expect. It was refreshing to witness the youngsters being refused sales in the majority of premises, but still disappointing that three premises were prepared to sell alcohol to teenagers who were clearly under-age.
"Retailers need to be aware that we could carry this type of initiative out at any time and if they want to avoid prosecution they need to make sure that are consistently checking ID and refusing sales to minors. Alcohol is a factor in a significant number of traffic and crime incidents and irresponsible sales of alcohol does nothing to help us combat that."
Following the operation the police are now taking action under liquor licensing laws.
Media enquiries should be referred to Constable Karl Williams on 06 327 6555. In Karl's absence speak to Sgt Peter Wood.